Semi-Quantitative Thickness Determination

ABSTRACT

While a substrate is polished, it is also irradiated with light from a light source. A current spectrum of the light reflected from the surface of the substrate is measured. A selected peak, having a first parameter value, is identified in the current spectrum. A value of a second parameter associated with the first parameter is determined from a lookup table using a processor. Depending on the value of the second parameter, the polishing of the substrate is changed. An initial spectrum of light reflected from the substrate before the polishing of the substrate can be measured and a wavelength corresponding to a selected peak of the initial spectrum can be determined.

BACKGROUND

The present invention relates to generally to chemical mechanicalpolishing of substrates.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually desired forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing diskpad or belt pad. The polishing pad can be either a standard pad or afixed abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry istypically supplied to the surface of the polishing pad. The polishingslurry includes at least one chemically reactive agent and, if used witha standard polishing pad, abrasive particles.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm leads to increased circuit resistance. On the other hand,underpolishing (removing too little) of a conductive layer leads toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

SUMMARY

While a substrate is polished, it is also irradiated with light from alight source. A current spectrum of the light reflected from the surfaceof the substrate is measured.

In one embodiment, a selected peak, having a first parameter value, isidentified in the current spectrum. A value of a second parameterassociated with the first parameter is determined from a lookup tableusing a processor. Depending on the value of the second parameter, thepolishing of the substrate is changed. An initial spectrum of lightreflected from the substrate before the polishing of the substrate canbe measured and a wavelength corresponding to a selected peak of theinitial spectrum can be determined.

In another embodiment, a polishing endpoint is determined, using aprocessor, from a lookup table, wherein the lookup table includes valuesfor a first parameter and a second parameter, and the polishing endpointis determined by defining a target value of the second parameter anddetermining a value of the first parameter associated with the targetvalue of the second parameter. The substrate is polished and thesubstrate is irradiated with a second beam of light. A current spectrumof the second beam of light reflected from the surface of the substratewhile the substrate is being polished is measured. A selected peak inthe current spectrum and a wavelength corresponding to the selected peakis identified. A value of the first parameter associated with theselected peak is determined. If the value of the first parameterassociated with the selected peak is within a predetermined range of thevalue of the first parameter associated with the target value of thesecond parameter, the polishing is ceased.

Polishing a substrate can comprise removing a quantity of a planarizingsubstance from a surface of the substrate, and the second parameter is athickness of the planarizing substance.

The first parameter can be a wavelength shift relative to the wavelengthcorresponding to the selected peak of the initial spectrum. The firstparameter can further be a change of width of the selected peak withrespect to the width of the selected peak of the initial spectrum.

A process associated with the polishing can be a shallow trenchisolation process. A process associated with the polishing can furtherbe a interlayer dielectric process.

The various aspects of the invention may be embodied in processesperformed by data processing equipment, as a tangible computer readablemedium encoded with computer program instructions that instruct acomputer to perform such a process, or as data processing equipment thatperforms such a process.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a chemical mechanical polishing apparatus.

FIGS. 2A-2H show implementations of a polishing pad window.

FIG. 3 shows an implementation of a flushing system.

FIG. 4 shows an alternative implementation of the flushing system.

FIG. 5 is an overhead view of a polishing pad and shows locations wherein-situ measurements are taken.

FIGS. 6A-D shows a plot of a spectrum obtained from in-situmeasurements.

FIG. 7 shows a plot of the location of a selected peak in a spectrum vs.oxide thickness.

FIG. 8 shows a lookup table.

FIG. 9 shows a flow chart illustrating the use of a lookup table inchanging a polishing of a substrate by using a value of a secondparameter associated with a selected peak.

FIG. 10 shows a flow chart illustrating the use of a lookup table inendpoint determination.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a polishing apparatus 20 operable to polish a substrate 10.The polishing apparatus 20 includes a rotatable disk-shaped platen 24,on which a polishing pad 30 is situated. The platen is operable torotate about axis 25. For example, a motor can turn a drive shaft 22 torotate the platen 24. The polishing pad 30 can be detachably secured tothe platen 24, for example, by a layer of adhesive. When worn, thepolishing pad 30 can be detached and replaced. The polishing pad 30 canbe a two-layer polishing pad with an outer polishing layer 32 and asofter backing layer 34.

Optical access 36 through the polishing pad is provided by including anaperture (i.e., a hole that runs through the pad) or a solid window. Thesolid window can be secured to the polishing pad, although in someimplementations the solid window can be supported on the platen 24 andproject into an aperture in the polishing pad. The polishing pad 30 isusually placed on the platen 24 so that the aperture or window overliesan optical head 53 situated in a recess 26 of the platen 24. The opticalhead 53 consequently has optical access through the aperture or windowto a substrate being polished. The optical head is further describedbelow.

The window can be, for example, a rigid crystalline or glassy material,e.g., quartz or glass, or a softer plastic material, e.g., silicone,polyurethane or a halogenated polymer (e.g., a fluoropolymer), or acombination of the materials mentioned. The window can be transparent towhite light. If a top surface of the solid window is a rigid crystallineor glassy material, then the top surface should be sufficiently recessedfrom the polishing surface to prevent scratching. If the top surface isnear and may come into contact with the polishing surface, then the topsurface of the window should be a softer plastic material. In someimplementations the solid window is secured in the polishing pad and isa polyurethane window, or a window having a combination of quartz andpolyurethane. The window can have high transmittance, for example,approximately 80% transmittance, for monochromatic light of a particularcolor, for example, blue light or red light. The window can be sealed tothe polishing pad 30 so that liquid does not leak through an interfaceof the window and the polishing pad 30.

In one implementation, the window includes a rigid crystalline or glassymaterial covered with an outer layer of a softer plastic material. Thetop surface of the softer material can be coplanar with the polishingsurface. The bottom surface of the rigid material can be coplanar withor recessed relative to the bottom surface of the polishing pad. Inparticular, if the polishing pad includes two layers, the solid windowcan be integrated into the polishing layer, and the bottom layer canhave an aperture aligned with the solid window.

Assuming that the window includes a combination of a rigid crystallineor glassy material and a softer plastic material, no adhesive need beused to secure the two portions. For example, in one implementation, noadhesive is used to couple the polyurethane portion to the quartzportion of the window. Alternatively, an adhesive that is transparent towhite light can be used or an adhesive can be applied so that lightpassing through the window does not pass through the adhesive. By way ofexample, the adhesive can be applied only to the perimeter of theinterface between the polyurethane and quartz portion. A refractiveindex gel can be applied to a bottom surface of the window.

A bottom surface of the window can optionally include one or morerecesses. A recess can be shaped to accommodate, for example, an end ofan optical fiber cable or an end of an eddy current sensor. The recessallows the end of the optical fiber cable or the end of the eddy currentsensor to be situated at a distance, from a substrate surface beingpolished, that is less than a thickness of the window. With animplementation in which the window includes a rigid crystalline portionor glass like portion and the recess is formed in such a portion bymachining, the recess is polished so as to remove scratches caused bythe machining. Alternatively, a solvent and/or a liquid polymer can beapplied to the surfaces of the recess to remove scratches caused bymachining. The removal of scratches usually caused by machining reducesscattering and can improve the transmittance of light through thewindow.

FIG. 2A-2H show various implementations of the window. As shown in FIG.2A, the window can have two portions, a polyurethane portion 202 and aquartz portion 204. The portions are layers, with the polyurethaneportion 202 situated on top of the quartz portion 204. The window can besituated in the polishing pad so that the top surface 206 of thepolyurethane layer is coplanar with a polishing surface 208 of thepolishing pad.

As shown in FIG. 2B, the polyurethane portion 202 can have a recess inwhich the quartz portion is situated. A bottom surface 210 of the quartzportion is exposed.

As shown in FIG. 2C, the polyurethane portion 202 can includeprojections, for example, projection 212, that project into the quartzportion 204. The projections can act to reduce the likelihood that thepolyurethane portion 202 will be pulled away from the quartz portion 204due to friction from the substrate or retaining ring.

As shown in FIG. 2D, the interface between the polyurethane portion 202and quartz portion 204 can be a rough surface. Such a surface canimprove the strength of the coupling of the two portions of the window,also reducing the likelihood the polyurethane portion 202 will be pulledaway from the quartz portion 204 due to friction from the substrate orretaining ring.

As shown in FIG. 2E, the polyurethane portion 202 can have non-uniformthickness. The thickness at a location that would be in the path 214 ofa light beam is less than the thickness at a location that would not bein the path 214 of the light beam. By way of example, thickness t₁ isless than thickness t₂. Alternatively, the thickness can be less at theedges of the window.

As shown in FIG. 2F, the polyurethane portion 202 can be attached to thequartz portion 204 by use of an adhesive 216. The adhesive can beapplied so that it would not be in the path 214 of the light beam.

As shown in FIG. 2G, the polishing pad can include a polishing layer anda backing layer. The polyurethane portion 202 extends through thepolishing layer and at least partially into the backing layer. The holein the backing layer can be larger in size than the hole in thepolishing layer, and the section of the polyurethane in the backinglayer can be wider than the section of the polyurethane in the polishinglayer. The polishing layer thus provides a lip 218 which overhangs thewindow and which can act to resist a pulling of the polyurethane portion202 away from the quartz portion 204. The polyurethane portion 202conforms to the holes of the layers of the polishing pad.

As shown in FIG. 2H, refractive index gel 220 can be applied to thebottom surface 210 of the quartz portion 204 so as to provide a mediumfor light to travel from a fiber cable 222 to the window. The refractiveindex gel 220 can fill the volume between the fiber cable 222 and thequartz portion 204 and can have a refractive index that matches or isbetween the indices of refraction of the fiber cable 222 and the quartzportion 204.

In implementations where the window includes both quartz andpolyurethane portions, the polyurethane portion should have a thicknessso that, during the life time of the polishing pad, the polyurethaneportion will not be worn so as to expose the quartz portion. The quartzcan be recessed from the bottom surface of the polishing pad, and thefiber cable 222 can extend partially into the polishing pad.

The above described window and polishing pad can be manufactured using avariety of techniques. The polishing pad's backing layer 34 can beattached to its outer polishing layer 32, for example, by adhesive. Theaperture that provides optical access 36 can be formed in the pad 30,e.g., by cutting or by molding the pad 30 to include the aperture, andthe window can be inserted into the aperture and secured to the pad 30,e.g., by an adhesive. Alternatively, a liquid precursor of the windowcan be dispensed into the aperture in the pad 30 and cured to form thewindow. Alternatively, a solid transparent element, e.g., the abovedescribed crystalline or glass like portion, can be positioned in liquidpad material, and the liquid pad material can be cured to form the pad30 around the transparent element. In either of the later two cases, ablock of pad material can be formed, and a layer of polishing pad withthe molded window can be scythed from the block.

With an implementation in which the window includes a crystalline orglass-like first portion and a second portion made of soft plasticmaterial, the second portion can be formed in the aperture of the pad 30by applying the described liquid precursor technique. The first portioncan then be inserted. If the first portion is inserted before the liquidprecursor of the second portion is cured, then curing can bond the firstand second portions. If the first portion is inserted after the liquidprecursor is cured, then the first and second potions can be secured byusing an adhesive.

The polishing apparatus 20 can include a flushing system to improvelight transmission through the optical access 36. There are differentimplementations of the flushing system. With implementations of thepolishing apparatus 20 in which the polishing pad 30 includes anaperture instead of a solid window, the flushing system is implementedto provide a laminar flow of a fluid, e.g., a gas or liquid, across atop surface of the optical head 53. (The top surface can be a topsurface of a lens included in the optical head 53.) The laminar flow offluid across the top surface of the optical head 53 can sweep opaqueslurry out of the optical access and/or prevent slurry from drying onthe top surface and, consequently, improves transmission through theoptical access. With implementations in which the polishing pad 30includes a solid window instead of an aperture, the flushing system isimplemented to direct a flow of gas at a bottom surface of the window.The flow of gas can prevent condensation from forming at the solidwindow's bottom surface which would otherwise impede optical access.

FIG. 3 shows an implementation of the laminar-flow flushing system. Theflushing system includes a gas source 302, a delivery line 304, adelivery nozzle 306, a suction nozzle 308, a vacuum line 310, and avacuum source 312. The gas source 302 and vacuum source can beconfigured so that they can introduce and suction a same or a similarvolume of gas. The delivery nozzle 306 is situated so that the laminarflow of gas is directed across the transparent top surface 314 of thein-situ monitoring module and not directed at the substrate surfacebeing polished. Consequently, the laminar flow of gas does not dry outslurry on a substrate surface being polished, which can undesirablyaffect polishing.

FIG. 4 shows an implementation of the flushing system for preventing theformation of condensation on a bottom surface of the solid window. Thesystem reduces or prevents the formation of condensation at the bottomsurface of the polishing pad window. The system includes a gas source402, a delivery line 404, a delivery nozzle 406, a suction nozzle 408, avacuum line 410, and a vacuum source 412. The gas source 402 and vacuumsource can be configured so that they can introduce and suction a sameor a similar volume of gas. The delivery nozzle 406 is situated so thatthe flow of gas is directed at the bottom surface window in thepolishing pad 30.

In one implementation that is an alternative to the implementation ofFIG. 4, the flushing system does not include a vacuum source or line. Inlieu of these components, the flushing system includes a vent formed inthe platen so that the gas introduced into the space underneath thesolid window can be exhausted to a side of the platen or, alternatively,to any other location in the polishing apparatus that can toleratemoisture.

The above-described gas source and vacuum source can be located awayfrom the platen so that they do not rotate with the platen. In thiscase, a rotational coupler for conveying gas is included for each of thesupply line and the vacuum line.

Returning to FIG. 1, the polishing apparatus 20 includes a combinedslurry/rinse arm 39. During polishing, the arm 39 is operable todispense slurry 38 containing a liquid and a pH adjuster. Alternative,the polishing apparatus includes a slurry port operable to dispenseslurry onto polishing pad 30.

The polishing apparatus 20 includes a carrier head 70 operable to holdthe substrate 10 against the polishing pad 30. The carrier head 70 issuspended from a support structure 72, for example, a carousel, and isconnected by a carrier drive shaft 74 to a carrier head rotation motor76 so that the carrier head can rotate about an axis 71. In addition,the carrier head 70 can oscillate laterally in a radial slot formed inthe support structure 72. In operation, the platen is rotated about itscentral axis 25, and the carrier head is rotated about its central axis71 and translated laterally across the top surface of the polishing pad.

The polishing apparatus also includes an optical monitoring system,which can be used to determine a polishing endpoint as discussed below.The optical monitoring system includes a light source 51 and a lightdetector 52. Light passes from the light source 51, through the opticalaccess 36 in the polishing pad 30, impinges and is reflected from thesubstrate 10 back through the optical access 36, and travels to thelight detector 52.

A bifurcated optical cable 54 can be used to transmit the light from thelight source 51 to the optical access 36 and back from the opticalaccess 36 to the light detector 52. The bifurcated optical cable 54 caninclude a “trunk” 55 and two “branches” 56 and 58.

As mentioned above, the platen 24 includes the recess 26, in which theoptical head 53 is situated. The optical head 53 holds one end of thetrunk 55 of the bifurcated fiber cable 54, which is configured to conveylight to and from a substrate surface being polished. The optical head53 can include one or more lenses or a window overlying the end of thebifurcated fiber cable 54 (e.g., as shown in FIG. 3). Alternatively, theoptical head 53 can merely hold the end of the trunk 55 adjacent thesolid window in the polishing pad. The optical head 53 can hold theabove-described nozzles of the flushing system. The optical head 53 canbe removed from the recess 26 as required, for example, to effectpreventive or corrective maintenance.

The platen includes a removable in-situ monitoring module 50. Thein-situ monitoring module 50 can include one or more of the following:the light source 51, the light detector 52, and circuitry for sendingand receiving signals to and from the light source 51 and light detector52. For example, the output of the detector 52 can be a digitalelectronic signal that passes through a rotary coupler, e.g., a slipring, in the drive shaft 22 to the controller for the optical monitoringsystem. Similarly, the light source can be turned on or off in responseto control commands in digital electronic signals that pass from thecontroller through the rotary coupler to the module 50.

The in-situ monitoring module can also hold the respective ends of thebranch portions 56 and 58 of the bifurcated optical fiber 54. The lightsource is operable to transmit light, which is conveyed through thebranch 56 and out the end of the trunk 55 located in the optical head53, and which impinges on a substrate being polished. Light reflectedfrom the substrate is received at the end of the trunk 55 located in theoptical head 53 and conveyed through the branch 58 to the light detector52.

In one implementation, the bifurcated fiber cable 54 is a bundle ofoptical fibers. The bundle includes a first group of optical fibers anda second group of optical fibers. An optical fiber in the first group isconnected to convey light from the light source 51 to a substratesurface being polished. An optical fiber in the second group isconnected to received light reflecting from the substrate surface beingpolished and convey the received light to a light detector. The opticalfibers can be arranged so that the optical fibers in the second groupform an X-like shape that is centered on the longitudinal axis of thebifurcated optical fiber 54 (as viewed in a cross section of thebifurcated fiber cable 54). Alternatively, other arrangements can beimplemented. For example, the optical fibers in the second group canform V-like shapes that are mirror images of each other. A suitablebifurcated optical fiber is available from Verity Instruments, Inc. ofCarrollton, Tex.

There is usually an optimal distance between the polishing pad windowand the end of the trunk 55 of bifurcated fiber cable 54 proximate tothe polishing pad window. The distance can be empirically determined andis affected by, for example, the reflectivity of the window, the shapeof the light beam emitted from the bifurcated fiber cable, and thedistance to the substrate being monitored. In one implementation, thebifurcated fiber cable is situated so that the end proximate to thewindow is as close as possible to the bottom of the window withoutactually touching the window. With this implementation, the polishingapparatus 20 can include a mechanism, e.g., as part of the optical head53, that is operable to adjust the distance between the end of thebifurcated fiber cable 54 and the bottom surface of the polishing padwindow. Alternatively, the proximate end of the bifurcated fiber cableis embedded in the window.

The light source 51 is operable to emit white light. In oneimplementation, the white light emitted includes light havingwavelengths of 200-800 nanometers. A suitable light source is a xenonlamp or a xenon-mercury lamp.

The light detector 52 can be a spectrometer. A spectrometer is basicallyan optical instrument for measuring properties of light, for example,intensity, over a portion of the electromagnetic spectrum. A suitablespectrometer is a grating spectrometer. Typical output for aspectrometer is the intensity of the light as a function of wavelength.

Optionally, the in-situ monitoring module 50 can include other sensorelements. The in-situ monitoring module 50 can include, for example,eddy current sensors, lasers, light emitting diodes, and photodetectors.With implementations in which the in-situ monitoring module 50 includeseddy current sensors, the module 50 is usually situated so that asubstrate being polished is within working range of the eddy currentsensors.

The light source 51 and light detector 52 are connected to a computingdevice operable to control their operation and to receive their signals.The computing device can include a microprocessor situated near thepolishing apparatus, e.g., a personal computer. With respect to control,the computing device can, for example, synchronize activation of thelight source 51 with the rotation of the platen 24. As shown in FIG. 5,the computer can cause the light source 51 to emit a series of flashesstarting just before and ending just after the substrate 10 passes overthe in-situ monitoring module. Each of points 501-511 represents alocation where light from the in-situ monitoring module impinged uponand reflected off of the substrate 10. Alternatively, the computer cancause the light source 51 to emit light continuously starting justbefore and ending just after the substrate 10 passes over the in-situmonitoring module.

With respect to receiving signals, the computing device can receive, forexample, a signal that carries information describing a spectrum of thelight received by the light detector 52. FIG. 6A shows examples of aspectrum measured from light that is emitted from a single flash of thelight source and that is reflected from the substrate. Spectrum 602 ismeasured from light reflected from a product substrate. Spectrum 604 ismeasured from light reflected from a base silicon substrate (which is awafer that has only a silicon layer). Spectrum 606 is from lightreceived by the optical head 53 when there is no substrate situated overthe optical head 53. Under this condition, referred to in the presentspecification as a dark condition, the received light is typicallyambient light.

The computing device can process the above described signal, or aportion thereof, to determine an endpoint of a polishing step. Withoutbeing limited to any particular theory, the spectrum of light reflectedfrom the substrate 10 evolves as polishing progresses. FIGS. 6B, 6C, and6D provide examples of the evolution of the spectrum as polishing of afilm of interest progresses for different shallow trench isolation (STI)processes. More specifically, both the location of a specific peak and awidth associated with the specific peak change as the polishingprogresses.

Results of the peak location as the polishing progresses are summarizedin the plot in FIG. 7. Note that, for all the processes illustratedhere, the location of the reflectance peak behaves about linearly withrespect to, in this case, an active oxide thickness. Without beinglimited to any particular theory, it has been observed that thethickness of a film on the substrate being polished changesproportionally to the change in the active oxide thickness. In general,the peak location does not have to behave linearly with the thickness ofthe film on the substrate being polished, but only need to behave in arelatively simple and known way, e.g., as a polynomial or as a set ofBezier functions.

Such a simple behavior belies possibly massive efforts that can berequired to compare the peak behavior of current spectra to thosegenerated either theoretically or experimentally in a reference library,as is done in copending application U.S. Ser. No. 11/555,171. Rather,because the behavior of parameters associated with the reflectance peaksin a given process is simple, much of this effort can be bypassed bycapturing the relationship between the parameters associated with thepeak and a second parameter in a lookup table. The second parameter canrepresent, for example, the thickness of the film on the substrate beingpolished.

Such a lookup table is presented in FIG. 8. The lookup table is a listof parameter values. For example, a first parameter would be a change inpeak location relative to an initial spectrum. A corresponding secondparameter could then be the amount of polishing film removed. The lookuptable would be used as follows: if the selected peak of a currentspectrum is shifted by −247.00 Å, then the corresponding amount ofpolishing film removed would be 300.00 Å. Alternatively, if the selectedpeak of a current spectrum is shifted by −265.00 Å, then thecorresponding amount of polishing film removed would be 320.00 Å.

A more likely scenario, however, if that a measured value of the firstparameter is not exactly equal to a value of the first parameter in thelookup table. Assuming that the value of the first parameter lies inbetween the minimum and maximum value of the first parameter in thelookup table, then the corresponding value of the second parameter canbe found through an interpolation scheme. The interpolation schemedepends on the assumed behavior of the second parameter with respect tothe first parameter. For example, if the behavior is linear, then alinear interpolation would be performed. In that regard, if the selectedpeak of a current spectrum is shifted by −256.00 Å, then an assumptionof linear behavior would produce a value of the corresponding amount ofpolishing film removed of 310.00 Å. A different assumed behavior wouldrequire a different interpolation scheme which could produce a differentvalue of the corresponding amount of polishing film removed.

The lookup table may therefore be used to determine, at any given time,the amount of polishing film removed from a measured current reflectancespectrum. Such a method 900 to perform this determination is presentedin FIG. 9. The method 900 comprises polishing a substrate 902. Thepolishing is done with a polishing apparatus operable to polish thesubstrate. The polishing apparatus includes a rotatable disk-shapedplaten on which a polishing pad is situated.

As the substrate is being polished, a light from a light source 904 isgenerated. The light source is operable to emit white light. In oneimplementation, the white light emitted includes light havingwavelengths of 200-800 nanometers. A suitable light source is a xenonlamp or a xenon mercury lamp.

The light is normally reflected 906 off the substrate onto the detectorBecause the light spans over a range of wavelengths, a reflectancespectrum is formed at the detector. That is, the reflectance of thelight depends on the wavelength as, for example, noted in the plot inFIG. 6A. The reflectance spectrum can then be converted into anelectrical signal by the detector.

The signal is analyzed and a reflectance peak selected 908. A peak fromthe spectrum, which can include a plurality of peaks over the wavelengthrange, is then selected. A first parameter in the form of a relativechange in the wavelength and/or width of a peak (e.g., the widthmeasured at a fixed distance below the peak or measured at a heighthalfway between the peak and the nearest valley), an absolute wavelengthand/or width of the peak, or both can be used to determine the endpointfor polishing according to an empirical formula. The best peak (orpeaks) to use when determining the endpoint varies depending on whatmaterials are being polished and the pattern of those materials.

A method 908 for selecting a spectral peak to use when determining theendpoint for the polishing process is shown in FIG. 9. Properties of asubstrate with the same pattern as the product substrate are measured(step 922). The substrate which is measured is referred to in theinstant specification as a “set-up” substrate. The set-up substrate cansimply be a substrate which is similar to or the same as the productsubstrate, or the set-up substrate can be one substrate from a batch ofproduct substrates. The properties that are measured can include a prepolished thickness of a film of interest at a particular location ofinterest on the substrate. Typically, the thicknesses at multiplelocations are measured. The locations are usually selected so that asame type of die feature is measured for each location. Measurement canbe performed at a metrology station.

The set-up substrate is polished in accordance with a polishing step ofinterest and the spectra obtained during polishing are collected (step924). Polishing and spectral collection can be performed at the abovedescribed polishing apparatus. The spectra are collected by the in situmonitoring system during polishing. The substrate is overpolished, i.e.,polished past an estimated endpoint, so that the spectrum of the lightthat is reflected from the substrate when the target thickness isachieved can be obtained.

Properties of the overpolished substrate are measured (step 926). Theproperties include post polished thicknesses of the film of interest atthe particular location or locations used for the pre polishmeasurement.

The measured thicknesses and the collected spectra are used to select,by examining the collected spectra, a particular peak to monitor duringpolishing (step 928). The peak can be selected by an operator of thepolishing apparatus or the selection of the peak can be automated (e.g.,based on conventional peak finding algorithms and an empirical peakselection formula). If a particular region of the spectrum is expectedto contain a peak that is desirable to monitor during polishing (e.g.,due to past experience or calculations of peak behavior based ontheory), only peaks in that region need be considered. A peak typicallyis selected that exhibits a significant change in location or widthduring processing, particularly around the time that a target filmthickness is achieved. For example, peaks in a spectrum typically changesignificantly in location and width when an oxide layer is polished awayand an underlying nitride layer is exposed.

Once the peak has been selected, a second parameter is determined 910from a value of the first parameter and the lookup table as describedabove. The first parameter can be a change in wavelength of the peakfrom an initial wavelength before polishing, and the second parametercan be the amount of polishing film removed.

In response to a value of the second parameter determined from thelookup table, the polishing may be changed 912. For example, there maybe a critical thickness at which the polishing may need to be slowed orstopped.

The lookup table may be more specifically used for endpointdetermination during polishing. A method for accomplishing such adetermination is shown in FIG. 10. The method comprises the selecting1008 a peak of the spectrum, generating 1040 a value of a firstparameter associated with the selected peak, checking if the value ofthe first parameter associated with the selected peak of the spectrumcrosses a threshold associated with the polishing endpoint 1060, and, ifso, ceasing the polishing 1080.

Selecting 1008 a peak of the spectrum comprises measuring properties ofa setup substrate 1022 (described above in 922), determining 1024 apolishing endpoint associated with a target value of a second parameter,collecting 1026 spectra from the setup substrate (described above in924), measuring 1028 the polished setup substrate (described above in926), and selecting a peak to monitor (described above in 928).

Determining 1024 a polishing endpoint—that is, a value of a firstparameter associated with a target value of a second parameter—isperformed using the lookup table as described above.

Generating 1040 a value of the first parameter associated with theselected peak comprises polishing 1042 the substrate (described above in902), irradiating 1044 the substrate with light (described above in904), generating 1046 the current spectrum from reflected light at adetector (described above in 906), and determining 1048 a value of thesecond parameter associated with the selected peak (described above in908).

After the value of the first parameter is generated, it is thendetermined 1060 if this value crosses a threshold associated with thepolishing endpoint. This determination can be done using a processor.The processor can be located within a controller, the controllercomprising the detector. For example, if the second parameter is anamount of polishing film removed during polishing, and there isinitially, say, 300.00 Å of polishing film, then the polishing endpointis −247.00 Å. That is, when the value of the first parameter associatedwith the selected peak of the current spectrum crosses −247.00 Å, thenthe polishing ceases 1080. Otherwise, the polishing continues andfurther current spectra are generated 1040.

Embodiments of the invention and all of the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructural means disclosed in this specification and structuralequivalents thereof, or in combinations of them. Embodiments of theinvention can be implemented as one or more computer program products,i.e., one or more computer programs tangibly embodied in an informationcarrier, e.g., in a machine readable storage device or in a propagatedsignal, for execution by, or to control the operation of, dataprocessing apparatus, e.g., a programmable processor, a computer, ormultiple processors or computers. A computer program (also known as aprogram, software, software application, or code) can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file. A program can be stored in a portionof a file that holds other programs or data, in a single file dedicatedto the program in question, or in multiple coordinated files (e.g.,files that store one or more modules, sub programs, or portions ofcode). A computer program can be deployed to be executed on one computeror on multiple computers at one site or distributed across multiplesites and interconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the substrate. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems, e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly. Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and substrate can beheld in a vertical orientation or some other orientation.

Particular embodiments of the invention have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results.

1. A method, comprising: polishing a substrate with a polishing pad;irradiating the substrate with light from a light source; measuring,using a processor, a current spectrum of the light reflected from thesurface of the substrate while the substrate is being polished;identifying, using the processor, a selected peak in the currentspectrum and a wavelength corresponding to the selected peak, theselected peak having a first parameter value; determining, using theprocessor, a value of a second parameter associated with the firstparameter from a lookup table; and changing, using a controller, thepolishing of the substrate depending upon the value of the secondparameter.
 2. The method of claim 1, wherein polishing a substratecomprises removing a quantity of a planarizing substance from a surfaceof the substrate, and the second parameter is a thickness of theplanarizing substance.
 3. The method of claim 2, further comprising:measuring an initial spectrum of light reflected from the substratebefore the polishing of the substrate; and determining a wavelengthcorresponding to a selected peak of the initial spectrum.
 4. The methodof claim 3, wherein the first parameter is a wavelength shift relativeto the wavelength corresponding to the selected peak of the initialspectrum.
 5. The method of claim 3, wherein the first parameter is achange of width of the selected peak with respect to the width of theselected peak of the initial spectrum.
 6. The method of claim 1, whereina process associated with the polishing is a shallow trench isolationprocess.
 7. The method of claim 1, wherein a process associated with thepolishing process is an interlayer dielectric process.
 8. A method,comprising: irradiating a substrate with a first beam of light from alight source; measuring, using a processor, an initial spectrum of thefirst beam of light reflected from the substrate; determining, using aprocessor, a polishing endpoint from a lookup table, wherein the lookuptable includes values for a first parameter and a second parameter, andthe polishing endpoint is determined by: defining a target value of thesecond parameter; and determining a value of the first parameterassociated with the target value of the second parameter; polishing thesubstrate; irradiating a substrate with a second beam of light;measuring, using a processor, a current spectrum of the second beam oflight reflected from the surface of the substrate while the substrate isbeing polished; identifying, using a processor, a selected peak in thecurrent spectrum and a wavelength corresponding to the selected peak;determining, using a processor, a value of the first parameterassociated with the selected peak; and ceasing the polishing if thevalue of the first parameter associated with the selected peak is withina predetermined range of the value of the first parameter associatedwith the target value of the second parameter;
 9. The method of claim 8,wherein polishing the substrate comprises removing a quantity of aplanarizing substance from a surface of the substrate, and the secondparameter is a thickness of the planarizing substance.
 10. The method ofclaim 9, wherein the first parameter is a wavelength shift relative tothe wavelength corresponding to the selected peak of the initialspectrum.
 11. The method of claim 9, wherein the first parameter is achange of width of the selected peak with respect to the width of theselected peak of the initial spectrum.
 12. The method of claim 8,wherein the process a process associated with the polishing is a shallowtrench isolation process.
 13. The method of claim 8, wherein a processassociated with the polishing is an interlayer dielectric process.
 14. Acomputer program product, tangibly embodied on a computer-readablemedium, comprising instructions to cause a polishing system to performthe operations of: measuring a current spectrum of light from a lightsource reflected from the surface of the substrate while the substrateis being polished; identifying a selected peak in the current spectrumand a wavelength corresponding to the selected peak, the selected peakhaving a first parameter value; determining a value of a secondparameter associated with the first parameter from a lookup table; andchanging the polishing of the substrate depending upon the value of thesecond parameter.
 15. The computer program product of claim 14, whereinpolishing a substrate comprises removing a quantity of a planarizingsubstance from a surface of the substrate, and the second parameter is athickness of the planarizing substance.
 16. The computer program productof claim 15, further comprising: measuring an initial spectrum of lightreflected from the substrate before the polishing of the substrate; anddetermining a wavelength corresponding to a selected peak of the initialspectrum.
 17. The computer program product of claim 16, wherein thefirst parameter is a wavelength shift relative to the wavelengthcorresponding to the selected peak of the initial spectrum.
 18. Thecomputer program product of claim 16, wherein the first parameter is achange of width of the selected peak with respect to the width of theselected peak of the initial spectrum.
 19. The computer program productof claim 14, wherein a process associated with the polishing is ashallow trench isolation process.
 20. The computer program product ofclaim 14, wherein a process associated with the polishing is aninterlayer dielectric process.
 21. An apparatus comprising: a polishingpad configured to polish a substrate; a light source configured toirradiate the substrate with light; an in-situ optical monitoring systemconfigured to obtain a current spectrum; a computer-readable storagemedium configured to store a lookup table comprising values of a firstparameter and a second parameter; a controller configured to change thepolishing of the substrate based upon a value of the current spectrum;and a processor configured to measure the current spectrum from thelight reflected from the substrate, identify a selected peak in thecurrent spectrum, the selected peak having a first parameter value, anddetermine a value of the second parameter associated with the selectedpeak from the lookup table.
 22. The apparatus of claim 21, whereinpolishing a substrate comprises removing a quantity of a planarizingsubstance from a surface of the substrate, and the second parameter is athickness of the planarizing substance.
 23. The apparatus of claim 22,wherein the processor is further configured to: measure an initialspectrum of light reflected from the substrate before the polishing ofthe substrate; and determine a wavelength corresponding to a selectedpeak of the initial spectrum.
 24. The apparatus of claim 23, wherein thefirst parameter is a wavelength shift relative to the wavelengthcorresponding to the selected peak of the initial spectrum.
 25. Theapparatus of claim 23, wherein the first parameter is a change of widthof the selected peak with respect to the width of the selected peak ofthe initial spectrum.
 26. The apparatus of claim 21, wherein a processassociated with the polishing is a shallow trench isolation process. 27.The apparatus of claim 21, wherein a process associated with thepolishing is an interlayer dielectric process.